1. Field of the Invention
The present invention relates to a variable capacity wobble plate type compressor for a car air-conditioner, and more particularly, to a capacity control arrangement for controlling the discharge capacity of the variable capacity wobble plate type compressor.
2. Description of the Related Art
U.S. Pat. Nos. 4,730,986 and 4,747,754 disclose variable displacement or capacity wobble plate type compressors for a car air-conditioner provided with a capacity control unit employing a solenoid-operated valve to open and close a fluid passage communicating between a crankcase chamber for a wobble plate type rotation-to-reciprocation conversion mechanism and a discharge chamber for a high compressed refrigerant gas.
The conventional solenoid-operated valve of the variable capacity wobble plate type compressor is operated to fully open the fluid passage between the crankcase chamber and the discharge chamber, to thereby introduce a high pressure refrigerant gas from the discharge chamber into the crankcase chamber and apply a high back pressure to pistons when an engine load exceeds a predetermined level, for example, due to acceleration of the car. Upon application of the high back pressure to the pistons, a pulling force acts on a non-rotating wobble plate of the rotation-to-reciprocation conversion mechanism, connected to the pistons via piston rods, to reduce the inclination of the wobble plate with respect to a plane perpendicular to the axis of a drive shaft of the compressor, and as a result, the discharge capacity of the compressor is reduced to the lowest value.
In this conventional variable dispalcement wobble plate type compressor, the solenoid-operated valve remains fully open while the engine load is higher than a predetermined level, to maintain the compressor capacity at the lowest value, and thus the pressure within the crankcase chamber is maintained at a high level for a long period of time. As a result, a retainer ring for retaining the wobble plate on a rotatable drive plate keyed on the drive shaft, one end of each piston rod joined to the wobble plate by staking, the other end of each piston rod joined to the piston by staking, and other mechanical parts, must be exposed to an excessive mechanical load, which reduces the operational life of each of those parts.
To solve this problem, a copending U.S. patent application No. 07/306,342 of the same assignee as the present application (German Patent Application P 39033406) corresponding to Japanese Patent Application No. 63-26375 discloses a control circuit means for temporarily bringing the solenoid-operated valve of a variable capacity compressor, such as a variable capacity wobble plate type compressor, to a fully open condition, to thereby reduce the compressor capacity to a lowest value, and then reducing the opening of the solenoid-operated valve to an extent such that the lowest compressor capacity is maintained while the engine load is high. Nevertheless, in the control circuit means of the copending U.S. Patent Application, the temporary period of time for which the solenoid-operated valve is kept fully open is a fixed constant time, regardless of an extent of the cooling load of the car air-conditioner. Therefore, if the pressure in the crankcase chamber has been increased to a given high level according to a requirement for a cooling load reduction before the fully-open operation of the solenoid-operated valve is carried out, a state occurs wherein a high pressure level in the crankcase chamber is maintained for a time longer than the fixed constant period of time due to the fully opening of the solenoid-operated valve, and as a result, the parts of the compressor must be exposed to an excessive load in the same way as those of the conventional variable displacement wobble plate type compressors of U.S. Pat. Nos. 4,730,986 and 4,747,754.
According to the control circuit means of the copending U.S. patent application No. 07/306,342, an adverse state may also occur wherein the full opening of the solenoid valve for the fixed constant period of time is terminated before the compressor capacity is decreased to the lowest value, because the pressure in the crankcase chamber has been excessively lowered before the start of the full opening of the solenoid-operated valve. Consequently, the cooling ability of the car air-conditioner cannot be satisfactorily decreased even when the engine load is high, and therefore, the fuel consumption of the engine is increased.